Traveling grate stokers supply solid fuels to furnaces so that the thermal energy of the fuel may be absorbed by boiler tubes or other heat recovery apparatus. Such stokers operate by continuously charging a layer of coarse-sized coal on a traveling grate which may be either a chain grate or a bar grate, igniting the surface of the coal by radiation of heat from incandescent coal flame and refractory arches, perpetuating the combustion by forcing updraft through the surface-ignited layers of coal while it is being transported horizontally beneath boiler tubes or other heat recovery apparatus, terminating the combustion by continuous updraft and cooling of the residual ash, and discharging the ash of spent coal from the discharge end of the stoker.
A chain grate stoker consists of a series of chain links strung on rods in a staggered arrangement and moved by sprockets or drums, while a bar grate surface consists of rows of keys strung on bars which are in turn carried by chains driven by sprockets.
Bituminous coals are readily ignited because they immediately evolve gaseous vapors of condensable, volatile matter and gases of volatile matters which ignite at approximately 400.degree. F. and provide a highly luminous flame which transmits heat to refractories for reflection to the coal bed, as well as direct reflection of the luminous flame to the coal surface particles.
Difficultly ignited fuels generally contain lower quantities of combustible, volatile matter and hence do not evolve appreciable amounts of luminous flames for back radiation effect. Fuels, such as anthracite, coke breeze, chars, and a pelletized fuel such as the pellet fuel set forth in U.S. applications Ser. Nos. 763,226 and 898,798, field Jan. 27, 1977, and Apr. 24, 1978, respectively, are in the category of difficultly ignited fuels. Ordinarily, large, gently sloping, front arches and large rear arches constructed above the combustion bed of a traveling grate stoker are used to burn a difficultly ignited fuel. Most of the furnaces are provided with a rear arch and a relatively small front arch, but front arch and combination front and rear arch furnaces enjoy widespread use. Such refractory arches provide an abundance of radiation surfaces for reflecting heat and igniting the fuel and directing the draft toward the oncoming fuel for reignition. Examples of such stokers may be found in Comubstion Engineering, Glen R. Fryling, editor, Rev. Edition 2nd Impression; published by Combustion Engineering, Inc. 1967, chapter 18.